This invention relates to the separation of mixtures of solids according to the density of the solid components, particularly where the proportion of heavy solids to the total amount of solids is relatively small. It particularly relates to apparatus for separating heavy solids and light solids from mixtures thereof, which apparatus includes an air jig with a driven vibrating or "jigged" carrier and a means for separating the heavy fraction from the air jig which is contaminated with some light material, into a clean heavy fraction and a light fraction according to density.